<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>
<div id="app">
  <h2 class="title" :class="['activate','line']">{{message}}</h2>
  <h2 class="title" :class="[activate,line]">{{message}}</h2>
  <h2 class="title" :class="get_class()">{{message}}</h2>
</div>

<script src="../js/vue.js"></script>
<script>
  const app = new Vue({
    el:"#app",
    data: {
      message:"stupid mother fucker",
      activate:'aaaaa',
      line:'bbbbbb',
    },
    methods:{
      get_class:function (){
        return[this.activate,this.line]
      }
    }
  })
</script>
</body>
</html>